Sterilization system employing low frequency plasma

ABSTRACT

A method and system for sterilizing an article is provided that includes use of a low frequency (LF) gas discharge plasma. The method includes placing the article in a vacuum chamber and evacuating the vacuum chamber to a predetermined pressure. Gas or vapor species are introduced into the vacuum chamber, and a low frequency plasma is generated within the vacuum chamber, the low frequency plasma having a frequency of from 0 to approximately 200 kHz. The low frequency plasma is maintained for a time period sufficient to substantially remove gas or vapor species from the article. The sterilization system includes a vacuum chamber coupled to a vacuum pump and a vent, a first electrode, and a second electrode. The sterilization system further includes a second region within the vacuum chamber, the second region including a region between the first and second electrodes, and a first region within the vacuum chamber, the first region being in fluid communication with the second region. The sterilization system further includes a source of reactive agent species coupled to the vacuum chamber, a process control monitor, and a low frequency power module including components adapted to apply a low frequency voltage between the first electrode and second electrode to generate a low frequency plasma in the vacuum chamber, the low frequency voltage having a frequency of from 0 to approximately 200 kHz.

CLAIM OF PRIORITY

[0001] This application is a continuation of U.S. Utility patentapplication Ser. No. 09/676,919, filed Oct. 2, 2000, the disclosure ofwhich is hereby incorporated in its entirety by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to systems and methods for sterilizingarticles that include the use of a gas discharge plasma.

[0004] 2. Description of the Related Art

[0005] Plasmas produced using radio frequency (RF) generators inparticular have proven to be valuable tools in processes for thesterilization of medical devices. For example, in U.S. Pat. Nos.4,643,876 and 4,756,882, which are incorporated by reference herein,Jacobs, et al. disclose using hydrogen peroxide as a precursor in a lowtemperature sterilization system that employs RF plasma. The combinationof hydrogen peroxide vapor and a RF plasma provides an efficient methodof sterilizing medical devices without using or leaving highly toxicmaterials or forming toxic by-products. Similarly, Jacob, U.S. Pat. No.5,302,343, and Griffiths, et al., U.S. Pat. No. 5,512,244, teach the useof RF plasmas in a sterilization process.

[0006] However, there are problems associated with the use of an RFplasma in a sterilization process. The RF plasma may leave residualhydrogen peroxide on the sterilized article. The residual amount ofhydrogen peroxide remaining on the sterilized article depends upon theRF power applied to the article, the amount of time exposed to the RFplasma, and the material of the article. For example, while someplastics (e.g., polyurethane) absorb hydrogen peroxide, other materials(e.g., Teflon) absorb relatively little, thereby yielding less residualhydrogen peroxide after sterilization.

[0007] In addition, inherent inefficiencies in the energy conversionfrom the low frequency (e.g., 60 Hz) line voltage to the RF (e.g.,approximately 1 MHz-1 GHz) voltage used to generate the RF plasma limitthe power efficiency of such systems to typically less than 50%. Energyefficiency is further reduced by typically 5-20% by virtue of the lossesfrom the required impedance matching network between the RF generatorand the load. Such low energy efficiency significantly increases thecost per watt applied to the sterilized articles. The requiredinstrumentation for using RF electrical energy (e.g., RF generator,impedance matching network, monitoring circuitry) is expensive, whichalso increases the cost per watt applied to the sterilized articles.

SUMMARY OF THE INVENTION

[0008] One aspect of the present invention is a method of sterilizationof an article. The method comprises placing the article in a vacuumchamber and evacuating the vacuum chamber to a predetermined pressure.Gas or vapor species are introduced into the, vacuum chamber, and a lowfrequency plasma is generated within the vacuum chamber, the lowfrequency plasma having a frequency of from 0 to approximately 200 kHz.The low frequency plasma is maintained for a time period sufficient tosubstantially remove gas or vapor species from the article.

[0009] Another aspect of the present invention is a method ofsterilization of an article. The method comprises placing the article ina vacuum chamber and evacuating the vacuum chamber to a predeterminedpressure. A low frequency plasma is generated within the vacuum chamber,the low frequency plasma having a frequency of from 0 to approximately200 kHz. The low frequency plasma is maintained for a time periodsufficient to heat the article to aid the evaporation and removal ofwater and other absorbed gases from the vacuum chamber and the article.

[0010] Another aspect of the present invention is a system forsterilizing an article. This system comprises a vacuum chamber coupledto a vacuum pump and a vent, a first electrode, and a second electrode.The system further comprises a second region within the vacuum chamber,the second region comprising a region between the first and secondelectrodes. The system further comprises a first region within thevacuum chamber, the first region being in fluid communication with thesecond region. The system further comprises a source of fluid coupled tothe vacuum chamber, a process control monitor, and a low frequency powermodule comprising components adapted to apply a low frequency voltagebetween the first electrode and second electrode to generate a lowfrequency plasma in the vacuum chamber, the low frequency voltage havinga frequency of from 0 to approximately 200 kHz.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 schematically illustrates a preferred embodiment of asterilization system compatible with the present invention.

[0012]FIG. 2A schematically illustrates a preferred embodiment of acylindrically-shaped electrode with open ends and perforated sides.

[0013]FIG. 2B schematically illustrates an alternative embodiment of acylindrically-shaped electrode with open ends and louvered sides.

[0014]FIG. 2C schematically illustrates an alternative embodiment of acylindrically-shaped electrode with open ends and solid sides.

[0015]FIG. 2D schematically illustrates an alternative embodiment of anelectrode comprising one or more colinear cylindrically-shaped segmentswith open ends and solid sides.

[0016]FIG. 2E schematically illustrates an alternative embodiment of anelectrode with a partial cylindrical shape, open ends, and solid sides.

[0017]FIG. 2F schematically illustrates an alternative embodiment of acylindrically-symmetric and longitudinally-asymmetric electrode withopen ends and solid sides.

[0018]FIG. 2G schematically illustrates an alternative embodiment of oneor more asymmetric electrodes with open ends and solid sides.

[0019]FIG. 2H schematically illustrates an alternative embodiment of anelectrode system with a first electrode that is cylindrically-shapedwith open ends and solid sides, and a second electrode comprising a wiresubstantially colinear with the first electrode.

[0020]FIG. 2I schematically illustrates an alternative embodiment of agenerally square or rectangular electrode within a generally square orrectangular vacuum chamber.

[0021]FIG. 3, which is broken into FIGS. 3a and 3 b, schematicallyillustrates an embodiment of a low frequency power module compatiblewith the phase angle control method of the present invention.

[0022]FIG. 4, which is broken into FIGS. 4a and 4 b, schematicallyillustrates an embodiment of a low frequency power module compatiblewith the amplitude control method of the present invention.

[0023]FIG. 5A schematically illustrates the phase angle control methodof controlling the low frequency power applied to the plasma.

[0024]FIG. 5B schematically illustrates the amplitude control method ofcontrolling the low frequency power applied to the plasma.

[0025]FIG. 6 schematically illustrates a preferred embodiment of amethod of sterilization compatible with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] Production of gas discharge plasmas using low frequency (LF)voltages avoids the various problems inherent in the state of the artsterilization devices and processes which form and use plasmas producedby radio frequency (RF) voltages. First, LF plasma processing leavesless residual reactive species remaining on the sterilized articles thandoes RF plasma processing. Second, generation of the LF plasma is highlyenergy efficient because little or no frequency conversion from the linevoltage is needed. For example, by using no frequency conversion with aline voltage frequency of 60 Hz, the energy efficiency of thesterilization system can reach approximately 85-95%. Use of LF voltagesalso does not require an impedance matching network, thereby avoidingthe associated energy losses. Third, due to the simplifiedinstrumentation and higher energy efficiency of LF generation, the costper watt applied to the sterilized articles using LF plasmas can be aslow as one-tenth the cost per watt of using RF plasmas. Fourth, thesimplified instrumentation used for generating LF plasmas has proven tobe more reliable and robust, and requiring less complicated diagnosticinstrumentation.

[0027]FIG. 1 schematically illustrates one preferred embodiment of thepresent invention comprising a sterilization system 10. Thesterilization system 10 comprises a vacuum chamber 12, a vacuum pump 14,a vacuum pump line 15, a vacuum pump valve 16, a reactive agent source18, a reactive agent line 19, a reactive agent valve 20, a low frequency(LF) power module 22, an LF voltage conduit 24, a vent 26, a vent line27, a vent valve 28, a process control module 30, an electrode 32, and areactive agent monitor 34. Persons skilled in the art recognize thatother embodiments comprising sterilization systems of differentconfigurations than that illustrated in FIG. 1 are compatible with thepresent invention.

[0028] In the preferred embodiment of the present invention, thearticles (not shown in FIG. 1) to be sterilized are packaged in variouscommonly employed packaging materials used for sterilized products. Thepreferred materials are spunbonded polyethylene packaging materialcommonly available under the trademark “TYVEK” or composites of “TYVEK”with a polyethylene terephthalate packaging material commonly availableunder the trademark “MYLAR”. Other similar packaging materials may alsobe employed such as polypropylene. Paper packaging materials may also beused. With paper packaging, longer processing times may be required toachieve sterilization because of possible interactions of the reactiveagent with paper.

[0029] The vacuum chamber 12 of the preferred embodiment is sufficientlygas-tight to support a vacuum of approximately less than 40 Pa (0.3Torr). Coupled to the vacuum chamber 12 is a pressure monitor (notshown) which is also coupled to the process control module to provide ameasure of the total pressure within the vacuum chamber. Also coupled tothe vacuum chamber 12 is the reactive agent monitor 34 which is capableof detecting the amount of the reactive agent in the vacuum chamber 12.In the exemplary embodiment of the present invention, the reactive agentis hydrogen peroxide, and the reactive agent monitor 34 measures theabsorption of ultraviolet radiation at a wavelength characteristic ofhydrogen peroxide. Other methods of reactive agent detection compatiblewith the present invention include, but are not limited to, pressuremeasurement, near infrared absorption, and dew point measurement. Thereactive agent monitor 34 is also coupled to the process control module30 to communicate the detected amount of the reactive agent to theprocess control module 30.

[0030] In the preferred embodiment of the present invention, inside andelectrically isolated from the vacuum chamber 12 is the electrode 32,which is electrically conductive and perforated to enhance fluidcommunication between the gas and plasma species on each side of theelectrode 32. The electrode 32 of the preferred embodiment generallyconforms to the inner surface of the vacuum chamber 12, spacedapproximately one to two inches from the wall of the vacuum chamber 12,thereby defining a gap region between the vacuum chamber 12 and theelectrode 32. The electrode 32 is coupled to the LF power module 22 viathe LF voltage conduit 24. In the preferred embodiment, with the vacuumchamber 12 connected to electrical ground via a bypass capacitor andshunt resistor, application of an LF voltage between the vacuum chamber12 and the electrode 32 creates an LF electric field which is strongerin a second region 31 which includes the gap region and the vicinity ofthe edges of the electrode 32. The LF electric field is weaker in afirst region 33 where the sterilized articles are placed. Generally, inother embodiments, the LF electric field can be generated by applying anLF voltage between the electrode 32 and a second electrode in the vacuumchamber 12. In such embodiments, the second region 31 includes the gapregion between the two electrodes, and the vicinity of the edges of oneor both of the electrodes. The preferred embodiment in which the vacuumchamber 12 serves as the second electrode is one of the many differentways to generate the gas plasma.

[0031] In the preferred embodiment illustrated in FIG. 2A, acylindrically-shaped electrode 32 provides fluid communication betweenthe gas and plasma on each side of the electrode 32 through the openends of the electrode 32 as well as through the perforations in the sideof the electrode 32. These open ends and perforations permit gaseous andplasma species to freely travel between the second region 31 between theelectrode 32 and the walls of the vacuum chamber 12 and the first region33 where the sterilized articles are placed. Similarly, as illustratedin FIGS. 2B-2I, other configurations of the electrode 32 provide fluidcommunication between the second region 31 and the first region 33. FIG.2B schematically illustrates a cylindrically-shaped electrode 32 withopen ends and louvered openings along its sides. FIG. 2C schematicallyillustrates a cylindrically-shaped electrode 32 with open ends and solidsides. FIG. 2D schematically illustrates an electrode 32 comprising aseries of colinear cylindrically-shaped segments with open ends andsolid sides. FIG. 2E schematically illustrates an electrode 32 with apartial cylindrical shape, open ends and solid sides. FIG. 2Fschematically illustrates a cylindrically-symmetric andlongitudinally-asymmetric electrode 32 with open ends and solid sides.FIG. 2G schematically illustrates an asymmetric electrode 32 with openends and solid sides. More than one electrode can be used to generatethe plasma. FIG. 2H schematically illustrates an electrode system with afirst electrode 32 that is cylindrically-shaped with open ends and solidsides, and a second electrode 32′ comprising a wire substantiallycolinear with the first electrode 32. The LF voltage is applied betweenthe first electrode 32 and the second electrode 32′. In this embodiment,the second region 31 is the region between the first electrode 32 andthe second electrode 32′, and the first region 33 is between the firstelectrode 32 and the vacuum chamber 12. FIG. 21 schematicallyillustrates a generally square or rectangular electrode within agenerally square or rectangular vacuum chamber. The variousconfigurations for generally cylindrical electrodes schematicallyillustrated in FIGS. 2A-2H can also be applied to the generally squareor rectangular electrode of FIG. 21. Each of these embodiments of theelectrode 32 provide fluid communication between the second region 31and the first region 33.

[0032] The vacuum pump 14 of the preferred embodiment is coupled to thevacuum chamber 12 via the vacuum pump line 15 and the vacuum valve 16.Both the vacuum pump 14 and the vacuum pump valve 16 are coupled to, andcontrolled by, the process control module 30. By opening the vacuumvalve 16, gases within the vacuum chamber 12 are pumped out of thevacuum chamber 12 through the vacuum pump line 15 by the vacuum pump 14.In certain embodiments, the vacuum valve 16 is capable of being openedto variable degrees to adjust and control the pressure in the vacuumchamber 12.

[0033] The reactive agent source 18 of the preferred embodiment is asource of fluid coupled to the vacuum chamber 12 via the reactive agentline 19 and the reactive agent valve 20. The reactive agent valve 20 iscoupled to, and controlled by, the process control module 30. Thereactive agent source 18 of the preferred embodiment comprises reactiveagent species. In the preferred embodiment, the reactive agent speciescomprises a germicide which is a sterilant or a disinfectant, such ashydrogen peroxide. In addition, the germicide supplied by the reactiveagent source 18 can be in gas or vapor form. By opening the reactiveagent valve 20, reactive agent atoms and molecules from the reactiveagent source 18 can be transported into the vacuum chamber 12 via thereactive agent line 19. In certain embodiments, the reactive agent valve20 is capable of being opened to variable degrees to adjust the pressureof the reactive agent in the vacuum chamber 12. In the exemplaryembodiment of the present invention, the reactive agent species of thereactive agent source 18 comprising hydrogen peroxide molecules.

[0034] The vent 26 of the preferred embodiment is coupled to the vacuumchamber 12 via the vent line 27 and the vent valve 28. The vent valve 28is coupled to, and controlled by, the process control module 30. Byopening the vent valve 28, vent gas is vented into the vacuum chamber 12via the vent line 27. In certain embodiments, the vent valve 28 iscapable of being opened to variable degrees to adjust the pressure ofthe air in the vacuum chamber 12. In the exemplary embodiment of thepresent invention, the vent 26 is a High Efficiency Particulate-filteredAir (HEPA) vent which provides filtered air as the vent gas. Other ventgases compatible with the present invention include, but are not limitedto, dry nitrogen, and argon.

[0035] The process control module 30 is coupled to various components ofthe sterilization system 10 to control the sterilization system 10. Inan exemplary embodiment of the present invention, the process controlmodule 30 is a microprocessor configured to provide control signals tothe various other components in response to the various signals receivedfrom other components.

[0036] The LF power module 22 of the preferred embodiment is coupled tothe electrode 32 via the LF voltage conduit 24, and is coupled to, andcontrolled by, the process control module 30. The LF power module 22 isadapted to apply a low frequency voltage between the electrode 32 andthe vacuum chamber 12 so as to generate a low frequency plasma in thevacuum chamber 12. FIG. 3, which is broken into FIGS. 3a and 3 b,schematically illustrates an embodiment of the LF power module 22compatible with the phase angle control method of controlling the lowfrequency power applied to the plasma. As illustrated in FIG. 3, the LFpower module 22 comprises an over-power relay 40, a pair of metal oxidevaristors 42, a step-up transformer 50, a flyback current shunt element62, an inductor 64, a capacitor 66, and a LF power feedback controlsystem 70. The LF power feedback control system 70 illustrated in FIG. 3comprises a power controller 60, a current monitor 80, a voltage monitor90, and a power monitor 100 coupled to the current monitor 80 and thevoltage monitor 90. Line voltage (typically 200-240 VAC, 50/60 Hz) isprovided to the step-up transformer 50 via the closed over-power relay40 which is coupled to the LF power feedback control system 70. Forother frequencies, the LF power module 22 may also include a switchingmodule to provide lower frequencies or frequencies up to a few hundredkHz.

[0037] In the embodiment illustrated in FIG. 3, the metal oxidevaristors (MOVs) 42 are used to suppress transient voltage impulses.Each MOV 42 is a multiple-junction solid-state device capable ofwithstanding large magnitude impulses with a low amount of let-throughvoltage. The MOVs 42 serve as fast acting “variable resistors” with alow impedance at higher-than-normal voltages and a high impedance atnormal voltages. MOVs are manufactured for specific voltageconfigurations and for a variety of impulse magnitudes. Persons skilledin the art are able to select MOVs 42 consistent with the presentinvention.

[0038] The output voltage of the step-up transformer 50 is preferablybetween approximately 100 and 1000 V_(rms), more preferably betweenapproximately 200 and 500 V_(rms), and most preferably betweenapproximately 250 and 450 V_(rms). The output voltage of the step-uptransformer 50 is transmitted to the power controller 60, which providesthe LF voltage to the electrode 32 and vacuum chamber 12 via the flybackcurrent shunt element 62, the inductor 64, the capacitor 66, and the LFpower feedback control system 70. The flyback current shunt element 62provides a path for fly-back current and to tune the circuit, and in thepreferred embodiment the flyback current shunt element 62 is a loadresistor of approximately 1500 ohms. In other embodiments, the flybackcurrent shunt element 62 can be a snubber. The inductance of theinductor 64 is chosen to limit noise spikes in the LF current, and istypically approximately 500 mH. The capacitance of the capacitor 66 ischosen to maximize the efficiency of power transfer to the LF plasma bymatching the resonant frequency of the series LC circuit to thefrequency of the applied LF voltage. For a 60 Hz voltage and aninductance of 500 mH, a capacitance of approximately 13.6 μF providesthe resonant condition for which the impedance of the series LC circuitis approximately zero, thereby maximizing the transmitted LF power.Persons skilled in the art are able to select appropriate values forthese components depending on the frequency of the applied LF voltage ina manner compatible with the present invention.

[0039]FIG. 4, which is broken into FIGS. 4a and 4 b, schematicallyillustrates an embodiment of the LF power module 22 compatible with theamplitude control method of controlling the low frequency power appliedto the plasma. As illustrated in FIG. 4, the LF power module 22comprises an over-power relay 40, a pair of metal oxide varistors 42, astep-up transformer 55, and a LF power feedback control system 70. TheLF power feedback control system 70 illustrated in FIG. 4 comprises ahigh voltage (HV) DC power supply 51, a voltage-controlled oscillator(VCO) 52, a voltage-controlled amplifier (VCA) 53, a HV operationalamplifier 54, a current monitor 80, a voltage monitor 90, and a powermonitor 100 coupled to the current monitor 80 and the voltage monitor90. Line voltage is provided to the HV DC power supply 51 via the closedover-power relay 40 which is coupled to the LF power feedback controlsystem 70. The output of the HV DC power supply 51 is preferably betweenapproximately 100 and 1000 VDC, more preferably between approximately200 and 500 VDC, and most preferably between approximately 250 and 450VDC.

[0040] In the embodiment illustrated in FIG. 4, the VCO 52 generates asinewave output with a constant amplitude and fixed low frequency from 0to 1 MHz, the low frequency selected by supplying an appropriateset-point voltage to the VCO 52. Alternative embodiments can utilizeother waveforms, e.g., triangular or square waveforms. The LF output ofthe VCO 52 is supplied to the VCA 53, which serves as a power controllerto maintain a substantially stable average power applied to the lowfrequency plasma. In response to a feedback signal from the powercontrol module 110, the VCA 53 amplifies the LF output of the VCO 52 togenerate an amplified LF voltage with an amplitude between approximately0 and 12 VAC. The amplified LF voltage from the VCA 53 is supplied tothe HV operational amplifier 54 which in response generates a highvoltage LF output with an amplitude determined by the amplitude of theamplified LF voltage from the VCA 53. Appropriate HV operationalamplifiers are commercially available (e.g., Apex Microtechnology,Tuscon, Ariz., part number PA93), and persons skilled in the art areable to select a HV operational amplifier compatible with the presentinvention. Typically, the amplitude of the high voltage LF output fromthe HV operational amplifier 54 is approximately 100 to 150 VAC. Inorder to generate larger amplitude LF voltages to be applied to theplasma, the high voltage LF output from the HV operational amplifier 54can be further amplified by the step-up transformer 55, as illustratedin FIG. 4. Alternatively, the step-up transformer 55 may be omitted ifthe HV operational amplifier 54 is capable of generating a high voltageLF output with the desired amplitude to be applied to the plasma.

[0041] In both the phase angle control embodiment illustrated in FIG. 3and the amplitude control embodiment illustrated in FIG. 4, the LF powerfeedback control system 70 of the LF power module 22 further comprises apower control module 110 coupled to the power monitor 100, which iscoupled to the current monitor 80 and voltage monitor 90. The currentmonitor 80 measures the LF current through the electrode 32 and thevacuum chamber 12. In the preferred embodiment of the present invention,the current monitor 80 includes a current sensor 82 which provides avoltage output indicative of the measured real-time, cycle-by-cycle LFcurrent, a first converter 84 which produces a DC voltage in response tothe RMS of the voltage output of the current sensor 82, and a firstvoltage amplifier 86 which amplifies the DC voltage from the firstconverter 84 to produce a real-time current signal. In addition, thecurrent monitor 80 also includes an over-current detector 88, whichmonitors the DC voltage from the first converter 84 in real-time andsends an error signal to the power control module 110 if the LF currentexceeds a pre-set value, caused for example by a short circuit betweenthe electrode 32 and the vacuum chamber 12. Under such an occurrence,the LF voltage is turned off momentarily. This occurrence can result ina few cycles being lost, however the power is stabilized so that theaverage power is not affected by more than a predetermined tolerance.

[0042] The voltage monitor 90 measures the LF voltage between theelectrode 32 and the vacuum chamber 12. In the preferred embodiment ofthe present invention, the voltage monitor 90 includes a step-downtransformer 92 which produces a voltage output indicative of themeasured real-time, cycle-by-cycle LF voltage, a second converter 94which produces a DC voltage in response to the RMS of the voltage outputof the step-down transformer 92, and a second voltage amplifier 96 whichamplifies the DC voltage from the second converter 94 to produce areal-time voltage signal.

[0043] In the preferred embodiment, the power monitor 100 furthercomprises a multiplier that receives the DC voltages from the currentmonitor 80 and the voltage monitor 90, and multiplies these two voltagesto produce a real-time power signal proportional to the LF power appliedto the plasma between the electrode 32 and the vacuum chamber 12, thereal-time power signal being generated in response to the real-timecurrent and real-time voltage signals, and transmitted to the powercontrol module 110. In other embodiments, the power monitor 100 monitorsthe power applied to the plasma by utilizing a signal indicative of thereal-time impedance of the plasma with either the real-time current orreal-time voltage signals. In still other embodiments, the power monitor100 monitors the power applied to the plasma by utilizing otherreal-time signals which indirectly indicate the power applied to theplasma; e.g., a real-time signal proportional to the brightness of theglow discharge generated by the plasma. Persons skilled in the art canselect an appropriate power monitor 100 compatible with the presentinvention.

[0044] The power control module 110 of the preferred embodiment includesa fault detector, such as an over-power detector 112 which monitors thereal-time power signal from the power monitor 100 and opens theover-power relay 40 if the LF power exceeds a pre-set value, therebyextinguishing the LF plasma. After such an occurrence, the control ofrestart can be given to the user or to software. The power controlmodule 110 of the preferred embodiment further comprises an additionalfault detector, such as a thermal switch 114 which detects overheating,and a power control processor 120.

[0045] In the preferred embodiment, the power control processor 120controls and monitors the status of the LF power feedback control system70. The power control processor 120 is coupled to a user interface 122which provides user input regarding a selected power magnitude settingand a selected power on/off setting. The power control processor 120 isalso coupled to the power monitor 100, the thermal switch 114, and theover-current detector 88. In the preferred embodiment, the powermagnitude setting can be selected from two power levels, 800 W and 600W. When the power is turned on, the preferred embodiment of the powercontrol processor 120 ensures that a “soft start” condition ismaintained in which the inrush current is minimized. In addition, theuser interface 122 receives signals from the power control processor 120indicative of the status of the sterilization system 10, which iscommunicated to the user.

[0046] In the phase angle control embodiment illustrated in FIG. 3, thepower control processor 120 is also coupled to the power controller 60.In this embodiment, the power control processor 120 transmits a signalto the power controller 60 in response to signals from the userinterface 122, power monitor 100, over-current detector 88, and thermalswitch 114 in order to maintain a substantially stable LF power appliedto the LF plasma while avoiding error conditions. In the amplitudecontrol embodiment illustrated in FIG. 4, the power control processor120 is coupled to the VCA 53. In this embodiment, the power controlprocessor 120 transmits a signal to the VCA 53 in response to signalsfrom the user interface 122, power monitor 100, over-current detector88, and thermal switch 114 in order to maintain a substantially stableLF power applied to the LF plasma while avoiding error conditions. Inboth embodiments illustrated in FIG. 3 and FIG. 4, the power controlprocessor 120 typically maintains the LF power applied to the LF plasmawithin a tolerance of approximately 0-10% of the specified power level.

[0047] Note that not all of the components listed and described in FIG.3 and FIG. 4 are required to practice the present invention, since FIG.3 and FIG. 4 merely illustrate particular embodiments of the LF powermodule 22. These components include components for automation, safety,regulatory, efficiency, and convenience purposes. Other embodimentscompatible with the present invention can eliminate some or all of thesecomponents, or can include additional components.

[0048] In response to the signal from the power control processor 120,the power controller 60 of the embodiment illustrated in FIG. 3 controlsthe LF power applied between the electrode 32 and the vacuum chamber 12by utilizing phase angle control. Under phase angle control, the dutycycle of the LF power is modified by zeroing the voltage and currentapplied between the electrode 32 and the vacuum chamber 12 for a portionA of the cycle period. Such phase angle control is often used tomaintain constant power from electric heaters or furnaces. FIG. 5Aschematically illustrates the voltage and current for a 100% duty cycle(i.e., Δ=0) and for a reduced duty cycle (i.e., Δ≠0). During normaloperations, the power controller 60 maintains a constant LF powerapplied to the plasma by actively adjusting the duty cycle of the LFpower in response to the feedback real-time signal received from thepower control module 110 in response to the measured LF power. When afault event is detected by the over-current detector 88 or thermalswitch 114, the power control processor 120 reduces the LF power byreducing the duty cycle of the LF power, and it transmits a signal tothe user interface 122 to provide notification of the fault event.Persons skilled in the art are able to select appropriate circuitry tomodify the duty cycle of the LF power consistent with the presentinvention.

[0049] Alternatively, the LF power can be controlled by utilizingamplitude control, as in the embodiment illustrated in FIG. 4. Underamplitude control, the LF power is modified by adjusting the amplitudeof the voltage and current applied between the electrode 32 and thevacuum chamber 12. FIG. 5B schematically illustrates the voltage andcurrent corresponding to a first LF power setting and a second LF powersetting less than the first LF power setting. During normal operations,the VCA 53 maintains a constant LF power applied to the plasma byactively adjusting the amplitude of the LF power in response to thefeedback real-time signal received from the power control module 110 inresponse to the measured LF power. Persons skilled in the art are ableto select appropriate circuitry to modify the amplitude of the LF powerconsistent with the present invention.

[0050] The electronics for RF sterilizers are complicated by the need ofsuch systems to attempt to closely match the output impedance of the RFgenerator with the plasma impedance at all times in order to maximizepower efficiency and to avoid damage to the RF generator. Plasmaimpedance varies widely during plasma formation, being very high untilthe plasma is fully formed, and very low thereafter. When first ignitinga plasma, the RF generator cannot match the high plasma impedance whichexists prior to the full formation of the plasma, so a large fraction ofthe power output is reflected back to the RF generator. RF generatorshave protection systems which typically limit the RF generator outputduring periods of high reflected power to avoid damage. However, toignite the plasma, the voltage output of the RF generator must exceedthe threshold voltage required for plasma ignition. The thresholdvoltage is dependent on the chamber pressure, reactive agent, and otheroperating parameters and is approximately 300 V_(rms). In an RF system,once ignition has been achieved, and the plasma impedance is therebyreduced, the magnitude of the applied RF voltage must be reduced to asustaining voltage, e.g., approximately 140 V_(rms), to avoid excessivepower delivery. Because the higher RF voltages required for plasmaignition produce excessively high reflected power before full plasmaformation, RF generators require complicated safeguards to preventdamage during the plasma ignition stage.

[0051] Conversely, the complexity and rate of ignition failures aresignificantly reduced for LF sterilizers since the LF sterilizers mayoperate using applied voltages above the threshold voltage and have muchless restrictive output impedance matching requirements. During thetimes at which the applied LF voltage equals zero, as seen in FIG. 5A,the LF plasma is extinguished and there is no LF plasma in the vacuumchamber. The LF plasma must then be re-ignited twice each cycle. By onlyoperating in one voltage regime, LF sterilizers have simpler and morereliable electrical systems than do RF sterilizers. These electricalsystems are easier to service and diagnose, thereby reducing the costsassociated with repair. In addition, the higher peak plasma densitiesresulting from LF sterilizers likely result in increased dissociativerecombination on the articles, thereby reducing the amount of residualreactive species remaining on the articles after the sterilizationprocedure.

[0052]FIG. 6 schematically illustrates a preferred method ofsterilization using the apparatus schematically illustrated in FIG. 1.The sterilization process shown in FIG. 6 is exemplary, and personsskilled in the art recognize that other processes are also compatiblewith the present invention. The preferred process begins by sealing 200the article to be sterilized into the vacuum chamber 12. The vacuumchamber is then evacuated 210 by engaging the vacuum pump 14 and thevacuum valve 16 under the control of the process control module 30. Thevacuum chamber 12 is preferably evacuated to a pressure of less thanapproximately 660 Pa (5 Torr), more preferably between approximately 25to 270 Pa (0.2 to 2 Torr), and most preferably between approximately 40to 200 Pa (0.3 to 1.5 Torr).

[0053] In an exemplary process, upon reaching a desired pressure in thevacuum chamber 12, the process control module 30 signals the LF powermodule 22 to energize the electrode 32 within the vacuum chamber 12. Byapplying a LF voltage to the electrode 32, the LF power module 22ionizes the residual gases in the vacuum chamber 12, thereby creating220 a gas discharge LF plasma inside the vacuum chamber 12. This gasdischarge LF plasma is formed from the residual gases in the vacuumchamber 12, which are primarily air and water vapor. Because this gasdischarge LF plasma is created 220 before the reactive agent is injectedinto the vacuum chamber 12, this gas discharge LF plasma is typicallycalled the “pre-injection” plasma. The vacuum valve 14 is controllablyopened and closed to maintain a preset vacuum pressure during thepre-injection plasma step 220. The pre-injection plasma heats thesurfaces inside the vacuum chamber 12, including the articles, therebyaiding the evaporation and removal of condensed water and other absorbedgases from the vacuum chamber 12 and the articles. A similarpre-injection plasma is described by Spencer, et al. in U.S. Pat. Nos.5,656,238 and 6,060,019, which are incorporated by reference herein. Inan exemplary process, the pre-injection plasma is turned off afterapproximately 0 to 60 minutes. Other embodiments that are compatiblewith the present invention do not include the creation of thepre-injection plasma, or use multiple pre-injection plasmas. In stillother embodiments, the vacuum chamber 12 can be vented after thearticles are exposed to the pre-injection plasma.

[0054] In the preferred process, upon reaching a desired chamberpressure, the vacuum valve 16 is closed, and the reactive agent valve 20is opened under the control of the process control module 30, therebyinjecting 230 reactive agent from the reactive agent source 18 into thevacuum chamber 12 via the reactive agent line 19. In the preferredembodiment, the reactive agent comprises hydrogen peroxide, which isinjected in the form of a liquid which is then vaporized. The injectedliquid contains preferably from about 3% to 60% by weight of hydrogenperoxide, more preferably from about 20% to 60% by weight of hydrogenperoxide, and most preferably from about 40% to 60% by weight ofhydrogen peroxide. The concentration of hydrogen peroxide vapor in thevacuum chamber 12 may range from 0.125 to 20 mg of hydrogen peroxide perliter of chamber volume. The higher concentrations of hydrogen peroxidewill result in shorter sterilization times. Air or inert gas such asargon, helium, nitrogen, neon, or xenon may be added to the chamber withthe hydrogen peroxide to maintain the pressure in the vacuum chamber 12at the desired level. This injection 230 of reactive agent may occur asone or more separate injections.

[0055] Due to this injection 230 of reactive agent, the chamber pressureof the preferred process rises to approximately 2000 Pa (15 Torr) ormore. After approximately 6 minutes into the injection stage 230, thereactive agent is permitted to diffuse 240 completely and evenlythroughout the vacuum chamber 12. After approximately 1-45 minutes ofdiffusing 240, the reactive agent is substantially in equilibrium insidethe vacuum chamber 12. This diffusing 240 allows the reactive species todiffuse through the packaging material of the articles, and come intoclose proximity, if not contact, with the surfaces of the articles,thereby sterilizing the articles. In other embodiments, the diffusion ofthe reactive agent can be immediately followed by a vent of the vacuumchamber 12.

[0056] The vacuum chamber 12 is then partially evacuated 250 by pumpingout a fraction of the reactive agent from the vacuum chamber 12 bycontrollably opening the vacuum valve 16 under the control of theprocess control module 30. Once the vacuum pressure within the vacuumchamber 12 has reached the desired pressure, the vacuum valve 16 iscontrollably adjusted to maintain the desired pressure, and the processcontrol module 30 signals the LF power module 22 to energize theelectrode 32 within the vacuum chamber 12. In the preferred embodimentin which the reactive agent comprises hydrogen peroxide, the pressure ofthe hydrogen peroxide in the vacuum chamber 12 is preferably less thanapproximately 670 Pa (5 Torr), more preferably between approximately 25and 270 Pa (0.2 to 2 Torr), and most preferably between approximately 40and 200 Pa (0.3 to 1.5 Torr). By applying a LF voltage to the electrode32, the LF power module 22 generates 260 a reactive agent LF plasmainside the vacuum chamber 12 by ionizing the reactive agent. The articleis exposed to the reactive agent LF plasma for a controlled period oftime. In the preferred embodiment, an additional cycle 275 is performed.Other embodiments may omit this additional cycle 275, or may includefurther cycles.

[0057] In both RF and LF plasmas, the components of the reactive agentplasma include dissociation species of the reactive agent and moleculesof the reactive agent in excited electronic or vibrational states. Forexample, where the reactive agent comprises hydrogen peroxide as in thepreferred embodiment, the reactive agent plasma likely includes chargedparticles such as electrons, ions, various free radicals (e.g., OH,O₂H), and neutral particles such as ground state H₂O₂ molecules andexcited H₂O₂ molecules. Along with the ultraviolet radiation produced inthe reactive agent plasma, these reactive agent species have thepotential to kill spores and other microorganisms.

[0058] Once created, the charged particles of the reactive agent plasmaare accelerated by the electric fields created in the vacuum chamber 12.Because of the fluid communication between the second region 31 and thefirst region 33, some fraction of the charged particles created in thesecond region 31 are accelerated to pass from the second region 31 tothe first region 33 which contains the articles.

[0059] Charged particles passing from the second region 31 to the firstregion 33 have their trajectories and energies affected by the electricpotential differential of the sheath regions between the plasma and thewalls of the vacuum chamber 12 and the electrode 32. These sheathregions are created by all electron-ion plasmas in contact with materialwalls, due to charged particles impinging from the plasma onto thewalls. Electrons, with their smaller mass and hence greater mobility,are lost from the plasma to the wall before the much heavier and lessmobile ions, thereby creating an excessive negative charge densitysurrounding the walls and a corresponding voltage differential whichequalizes the loss rates of the electrons and the ions. This voltagedifferential, or sheath voltage, accelerates electrons away from thewall surface, and accelerates positive ions toward the wall surface.

[0060] The sheath voltage varies for different plasma types,compositions, and methods of production. For RF plasmas, the sheathvoltage is typically 40%-80% of the RF voltage applied to the electrode32. For example, for a root-mean-squared (RMS) RF voltage of 140 V_(rms)applied to the electrode 32 once the RF plasma is established, thecorresponding sheath voltage is approximately 55-110 V_(rms). An ionentering the sheath region surrounding the electrode 32 will then beaccelerated to an energy of 55-110 eV. This acceleration of positiveions by the sheath voltage is the basic principle behind semiconductorprocessing by RF plasmas.

[0061] As described above, for the LF plasmas of the preferredembodiment of the present invention, the voltage applied to theelectrode 32 may be equal to or greater than the ignition thresholdvoltage, which is typically 300 V_(rms). In addition, for LF plasmas,the sheath voltage is typically a higher percentage of the appliedvoltage than for RF plasmas, so the sheath voltage of the preferredembodiment of the present invention is then much higher than the sheathvoltage for an RF plasma system. This higher sheath voltage therebyaccelerates the charged particles of the LF plasma to much higherenergies. Therefore, because the charged particles are accelerated tohigher energies, the charged particles of the LF plasma of the preferredembodiment travel farther and interact more with the articles than dothe charged particles of RF plasma sterilizers.

[0062] Since the LF electric field changes polarity twice each cycle,the direction of the electric field acceleration on the chargedparticles reverses twice each cycle. For charged particles in the secondregion 31, this oscillation of the direction of the acceleration resultsin an oscillation of the position of the charged particles. However,because of the fluid communication between the second region 31 and thefirst region 33, some fraction of the charged particles are able to passto the first region 33 containing the articles from the second region 31before the direction of the electric field acceleration reverses.

[0063] The fraction of the charged particles created in the reactiveagent LF plasma which enter the first region 33 is a function of thefrequency of the applied electric field. The charged particles have twocomponents to their motion—random thermal speed and drift motion due tothe applied electric field. The thermal speed, measured by thetemperature, is the larger of the two (typically approximately 10⁷−10⁸cm/sec for electrons), but it does not cause the charged particles toflow in any particular direction. Conversely, the drift speed isdirected along the electric field, resulting in bulk flow of chargedparticles in the direction of the applied electric field. The magnitudeof the drift speed is approximately proportional to the magnitude of theapplied electric field, and inversely proportional to the mass of thecharged particle. In addition, the magnitude of the drift speed isdependent on the gas species and chamber pressure. For example, fortypical operating parameters of gas discharge plasma sterilizers,including an average electric field magnitude of approximately 1volt/cm, the drift speed for an electron formed in a gas dischargeplasma is typically approximately 106 cm/sec.

[0064] A charged particle enters the first region 33 containing thearticles only if it reaches the first region 33 before the polarity ofthe applied electric field changes, which would reverse the accelerationof the charged particle away from the electrode 32. For example, for anapplied RF electric field with a frequency of 13.56 MHz, the period ofthe electric field is approximately 7.4×10⁻⁸ sec, so an electron onlymoves a distance of approximately 3.7×10⁻³ cm during the half-cycle orhalf-period before the direction of the electric field changes and theelectron is accelerated away from the electrode 32. Due to their muchlarger masses, ions move much less than do electrons. Where the secondregion 31 between the vacuum chamber 12 and the electrode 32 isapproximately 2.54 cm wide, as in the preferred embodiment, only afraction of the charged particles created by an RF plasma would actuallyreach the first region 33 containing the articles.

[0065] Conversely, for an applied LF electric field with a frequency of60 Hz, the period of the electric field is approximately 16.7×10⁻³ sec,so an electron can move approximately 8.35×10³ cm before it isaccelerated away from the electrode 32. Therefore, the use of LFvoltages to create the plasma in the sterilization system 10 of thepreferred embodiment results in more activity in the first region 33, ascompared to a plasma generated using RF voltages. This higher activityin LF sterilizers likely contributes to the increased efficiency for theremoval of residual reactive species from the sterilized articles ascompared to RF sterilizers.

[0066] The plasma decay time, defined as a characteristic time for theplasma to be neutralized after power is no longer applied, provides anapproximate demarcation between the LF and RF regimes. The plasma decaytime is not known precisely, but it is estimated to be approximately10⁻⁴-10⁻³ sec for the plasma densities used in sterilizer systems, suchas the preferred embodiment of the present invention. This plasma decaytime corresponds to the time a charged particle exists before it isneutralized by a collision with a surface or another plasma constituent,and is dependent on the plasma species generated and the geometries ofthe various components of the sterilization system 10. As describedabove, the LF regime is characterized by a plasma which is extinguishedand re-ignited twice each cycle, i.e., the half-period of the applied LFvoltage is greater than the plasma decay time. Therefore, thesterilization system 10 is continually run at an applied voltage abovethe ignition threshold voltage of the plasma in order to re-ignite theplasma. The estimated approximate range of plasma decay times of10⁻⁴-10⁻³ sec for many of the plasmas compatible with the presentinvention then translates to an upper limit on the low frequency regimeof approximately 1-10 kHz. However, under certain circumstances, higherfrequencies can be tolerated.

[0067] Alternatively, the upper limit of the low frequency regime may bedefined as the frequency at which the electron drift speed is too slowfor an electron to traverse the 2.54-cm-wide second region 31 during ahalf-period of the applied LF voltage. Under typical operatinggeometries, this upper limit of the low frequency regime would beapproximately 200 kHz. For other geometries, the upper limit of the lowfrequency regime can be correspondingly different.

[0068] In the preferred embodiment of the present invention, thefrequency of the LF voltage applied to the plasma is preferably from 0to approximately 200 kHz, more preferably from 0 to approximately 10kHz, still more preferably from 0 to approximately 1 kHz, and even morepreferably from 0 to approximately 400 Hz. When selecting the frequencyof the LF voltage applied to the plasma, the frequency is mostpreferably selected to have a half-period greater than the plasma decaytime of the plasma.

[0069] In the preferred method, the LF power module 22 remains energizedfor approximately 2-15 minutes, during which the plasma removes excessresidual reactive species present on surfaces within the vacuum chamber12, including on the articles. There is a brief rise of the vacuumpressure upon generating 260 the plasma, however, the majority of theresidual removal step 270 is conducted at an approximately constantvacuum pressure of 50 to 70 Pa (0.4 to 0.5 Torr). The residual removalstep 270 is ended by the process control module 30, which turns off theLF power module 22, thereby quenching the plasma.

[0070] After the residual removal step 270, the vacuum chamber 12 isvented 280 by the process control module 30 which opens the vent valve28, thereby letting in vent gas from the vent 26 through the vent line27 and the vent valve 28. In the preferred process, the vacuum chamber12 is then evacuated 290 to a pressure of approximately 40 to 105 Pa(0.3 to 0.8 Torr) to remove any remaining reactive agent which may bepresent in the vacuum chamber 12. The vacuum chamber 12 is then ventedagain 300 to atmospheric pressure, and the sterilized articles are thenremoved 310 from the vacuum chamber 12.

[0071] The LF plasma provides a reduction of the amount of residualreactive agent molecules remaining on the articles after thesterilization procedure is complete. Where the reactive agent compriseshydrogen peroxide, the amount of residual hydrogen peroxide remaining onthe sterilized articles is preferably less than approximately 8000 ppm,more preferably less than approximately 5000 ppm, and most preferablyless than approximately 3000 ppm. In a comparison of the amount ofresidual hydrogen peroxide remaining after a LF plasma sterilization ascompared to a RF plasma sterilization, nine polyurethane test sampleswere exposed to hydrogen peroxide during a simulated sterilization cyclein both a LF sterilizer and a RF sterilizer. Each sample was prepared bywashing with Manuklenz® and drying prior to sterilization to avoid anycross contamination. The nine samples were then distributed uniformlyacross the top shelf of a standard industrial rack.

[0072] A full LF sterilization cycle, which matched nearly exactly theconditions of a standard RF sterilizer cycle, was used to perform thecomparison. The full LF sterilization cycle included a 20-minuteexposure to a pre-injection plasma, a first 6-minute hydrogen peroxideinjection, a vent to atmosphere, a 2-minute diffusion, a first 2-minutepost-injection plasma, a second 6-minute hydrogen peroxide injection, avent to atmosphere, a 2-minute diffusion, a second 2-minutepost-injection plasma, and a vent to atmosphere. Two full LFsterilization cycles were performed and compared to two full RFsterilization cycles. As seen in Table 1, all parameters other than thepost-injection plasma power were maintained as constant as possible fromrun to run. TABLE 1 LF Run 1 LF Run 2 RF Run 1 RF Run 2 Pre-injection727 W 779 W 751 W 752 W plasma power First post- 783 W 874 W 757 W 756 Winjection plasma power Second post- 755 W 893 W 758 W 758 W injectionplasma power Chamber temp. 45° C. nom. 45° C. nom. 45° C. nom. 45° C.nom. Injection 65-75° C. 65-75° C. 65-75° C. 65-75° C. system temp. H₂O₂17 mg/l 17 mg/l 17 mg/l 17 mg/l concentration Chamber 50 Pa 50 Pa 50 Pa50 Pa pressure during (0.4 Torr) (0.4 Torr) (0.4 Torr) (0.4 Torr) plasma

[0073] Variations in the pre-plasma power were ±3.5%, so the sampletemperature was approximately constant from run to run. The samples werethen removed and the residual analysis was performed.

[0074] The LF sterilizer used to generate the LF plasma was operated at60 Hz, and with an inductor of 500 mH and a capacitor of 13.6 μF. LFplasma power was determined by multiplying the voltage across the LFplasma by the current, then averaging on an oscilloscope. Thefluctuation level of the LF power was approximately 10%. Table 2illustrates the results of the comparison. TABLE 2 LF Run 1 LF Run 2 RFRun 1 RF Run 2 Average post- 769 W 884 W 757 W 757 W injection plasmapower H₂O₂ 1973 ± 144 1864 ± 75 2682 ± 317 2510 ± 203 residuals (ppm)

[0075] Exposure to a LF post-injection plasma reduced the residualreactive species more effectively than did exposure to a RFpost-injection plasma of comparable power. LF Run 1 had approximately23% less residual hydrogen peroxide than either RF Run 1 or RF Run 2,even though all had approximately the same post-injection plasma power.The LF processes therefore resulted in less residual hydrogen peroxidethan did the corresponding RF process.

[0076] The comparison of the two LF sterilization cycles illustratesthat increased plasma power results in a reduction of the hydrogenperoxide residuals. Furthermore, the variation between samples, asindicated by the standard deviation of the residual measurements, wassignificantly reduced in the LF process, thereby indicating an increaseduniformity as compared to the RF process.

[0077] Although described above in connection with particularembodiments of the present invention, it should be understood thedescriptions of the embodiments are illustrative of the invention andare not intended to be limiting. Various modifications and applicationsmay occur to those skilled in the art without departing from the truespirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method of sterilization of an article, themethod comprising: placing the article in a first region; introducinggas or vapor species into a second region; generating a plasma byapplying an applied electric field in the second region, the secondregion in fluid communication with the first region, the first regionhaving an electric field weaker than the applied electric field in thesecond region, the applied electric field having a frequency of lessthan 10 kHz; and maintaining the plasma for a time period sufficient tosubstantially remove gas or vapor species from the article.
 2. Themethod as defined in claim 1, wherein the gas or vapor species compriseshydrogen peroxide.
 3. The method as defined in claim 2, wherein anamount of hydrogen peroxide remaining on the article after maintainingthe plasma is less than approximately 8000 ppm.
 4. The method as definedin claim 1, wherein the applied electric field is applied in the secondregion by applying a voltage between two electrodes.
 5. The method asdefined in claim 1, wherein the voltage has a frequency from 0 toapproximately 1 kHz.
 6. The method as defined in claim 1, wherein thevoltage has a frequency from 0 to approximately 400 Hz.
 7. The method asdefined in claim 1, wherein the plasma has a plasma decay time and theapplied electric field has a half-period greater than said plasma decaytime.
 8. A system for sterilizing an article, the system comprising: afirst electrode; a second electrode; a first region comprising a regionbetween the first and second electrodes; a second region in fluidcommunication with the first region, the second region adapted toreceive the article; a source of reactive agent species, the source influid communication with the first region; a process control module; anda power module comprising components adapted to apply a voltage betweenthe first electrode and second electrode so as to generate a plasma inthe first region, the voltage having a frequency of less than 10 kHz. 9.The system as described in claim 8, wherein the reactive agent speciescomprises hydrogen peroxide.
 10. The system as described in claim 8,wherein the power module comprises a power controller, a flyback currentshunt element, a current monitor, a voltage monitor, an inductor, acapacitor, and a power control module.
 11. The system as described inclaim 10, wherein the power control module is coupled to the processcontrol module and the power controller, and the power controller isadapted to adjust the duty cycle of the voltage applied between thefirst and second electrodes in response to the power control module. 12.The system as described in claim 10, wherein the power control module iscoupled to the process control module and the power controller, and thepower controller is adapted to adjust the amplitude of the voltageapplied between the first and second electrodes in response to the powercontrol module.
 13. The system as described in claim 10, wherein theinductor and capacitor comprise an LC circuit connected in series withthe power controller.
 14. The system as described in claim 13, whereinan inductance and a capacitance of the LC circuit are chosen to match aresonant frequency of the LC circuit to the frequency of the voltageapplied between the first and second electrodes.
 15. The system asdescribed in claim 8, wherein the voltage is above a threshold voltagerequired to ignite a plasma.
 16. The system as described in claim 8,wherein the voltage has a frequency from 0 to approximately 1 kHz. 17.The system as described in claim 8, wherein the voltage has a frequencyfrom 0 to approximately 400 Hz.
 18. The system as defined in claim 8,wherein the plasma has a plasma decay time and the voltage has ahalf-period greater than said plasma decay time.
 19. The system asdescribed in claim 8, wherein the power module further comprises aswitching module that in response to an input voltage provides an outputvoltage with a frequency different from the frequency of the inputvoltage.